1. Field of the Invention
The present invention relates to a filter circuit and a communication apparatus.
2. Description of the Related Art
As the scaling-down of a complementary metal-oxide semiconductor (CMOS) process, a power supply voltage of a radio frequency (RF) circuit is on the decrease. If an RF circuit is mounted by a circuit technique used hitherto under circumstances where the CMOS process is scaled down, there is a problem that a voltage margin is insufficient and the dynamic range of a signal amplitude is narrow. On the other hand, because the cutoff frequency of a transistor increases by the scaling-down of the CMOS process, there is an advantage that it is suitable for an operation of performing high-speed switching accurately with respect to time. There is also an advantage that the capacitance ratio is exact because the accuracy of lithography increases.
In order to gain the above advantages by avoiding the problem caused by the scaling-down of the CMOS process, novel technology that applies the concept of discrete-time signal processing to the RF circuit is developed, which is digital RF technology. A major circuit in the digital RF technology is a charge domain filter. The charge domain filter is a filter circuit that is composed of a transconductance amplifier, switches and capacitors. The charge domain filter performs sampling of an analog signal by conducting charge and discharge in synchronization with a clock and thereby performs filtering, decimation or the like by discrete-time signal processing. Such technology is disclosed in Japanese Unexamined Patent Application Publications Nos. 2008-17220 and 2009-27389, for example.